The present invention relates to a new and improved process for the preparation of nicotinic acid by the direct oxidation of .beta.-picoline in the gas phase, wherein water and .beta.-picoline are fed separately to the catalyst bed and the catalyst is based on a titanium dioxide support which has been prepared by the sulfate method and has a high specific surface area and a vanadium oxide content of from 5 to 50%.
Nicotinic acid is widely used in the fields of medicine and agriculture, both as a vitamin and as an intermediate for pharmaceuticals and plant growth regulators.
Various processes are known for the synthesis of nicotinic acid from .beta.-picoline. One of those processes is liquid phase direct oxidation using HNO.sub.3 and H.sub.2 SO.sub.4 (U.S. Pat. No. 2,586,555, 1952) at temperatures of from 75 to 300.degree. C. and with yields of from 66 to 77%. Disadvantages of those processes are the high salt production that results, as well as the production of large streams of waste water. A microbiological process for oxidizing .beta.-picoline to nicotinic acid (EP 442430, 1995) achieves a yield of 50% after a reaction time of 16 hours, the unsatisfactory space-time yield and the costly separation of the biomass from the nicotinic acid making industrial application appear disadvantageous.
In the gas phase, the ammoxidation of .beta.-picoline to 3-cyanopyridine with subsequent hydrolysis to nicotinic acid is known (USSR Inventor's Certificate No. 235764, 1969). A disadvantage of that process is that two process steps are required, and the nicotinic acid must additionally be separated from the product mixture by means of crystallization. A total nicotinic acid yield of from 86 to 88% is mentioned. Likewise in the gas phase, there are several investigations into the direct oxidation of .beta.-picoline using vanadium oxide catalysts. The best results mentioned are nicotinic acid yields of from 82 to 86% with the addition of air and water and at temperatures of from 250 to 290.degree. C. (EP 747 359, WO 95/20577, 1995). The advantages of the latter process variant are that auxiliary substances and solvents are not required, with the exception of the non-critical substances water and air for the addition of oxygen. However, the yields of nicotinic acid are still markedly less than 90%. Accordingly, that process is not sufficiently economical.
An object of the present invention was, therefore, further to improve the direct oxidation of .beta.-picoline to nicotinic acid and to achieve yields of .gtoreq.90%, in order markedly to improve the economics of the process.